Electron discharge devices



Dec. 15, 1959 R. w. HUNTER 2,917,653

ELECTRON DISCHARGE DEVICES Filed Oct. 16, 1956 F IG. 4

IN V EN TOR. ROGER W HUNTER BY 4 g j ATTORNEY? United States Patent Qflice 2,917,655; ELECTRON DISCHARGE DEVICES Roger W. Hunter, Fort Wayne, Ind., assiguor to International Telephone and Telegraph Corporation Application October 16, 1956, Serial No. 616,293

Claims. c1. $15- 13 This invention relates to electron discharge devices and is particularly directed to devices with an extended target area and an electron gun for uniformly bombarding or flooding the area.

In a number of display tubes, the electron image of a video signal is Written on a planar storage target screen electrode and the image is then transferred to a closely spaced phosphor anode by a low density flood beam of electrons. For a shadow-free visual image, it is necessary that the flood beam impinge upon the screen normal to the plane of the screen and that the density of electrons beuniform throughout the target screen area.

For practical reasons, the cathode for supplying the flood beam is small in size compared to the screen area to be served. In fact, in some tubes the electron source may be considered a point source of electrons.

For reasonable screen-to-cathode spacing within an evacuated envelope, the solid angle at the cathode subtended by the screen is necessarily quite wide. A wide solid angle, inturn, leads to a serious problem of spherical aberration. Shading or uneven illumination of the phosphor anode clearly indicates the spherical aberration and non-uniform angles of approach of the electrons at the surface of the storage target screen. Angles less than ninety degrees (90") occur first at the periphery of the screen when the gun'is spaced along a line perpendicular to the center of the screen. Attempts heretofore to collimate the flood beam has left much to be desired.

The object of this invention is an improved display tube with means for providing a flood beam which is uniform in density and is well collimated throughout the planar area to be flooded.

The objects of this invention are attained in a display tube havinga planar target screen electrode, a writing gun for scanning and writing an electron image on the planar electrode and a flood gun spaced from the center of the electrode on a line substantially perpendicular to the center of the electrode for flooding the planar electrode with electrons, characterized in that a negative electron lensconcentric with said line is disposed adjacent said floodgunfor diverging the beam of the gun, and a double positive electron lens concentric with said line is disposed between the'negativelens and said planar electrode for converging the electrons to paths normalto the electrode.

Thejabove-mentioned andother features and objects of this invention and the manner of attaining them will become more-apparent and the invention itself will be bestunderstood by reference to the following description of an embodiment ofthe invention taken in conjunction withthe accompanying drawings, wherein:

Fig.1 'isa longitudinal sectional view of one tube em bodying this invention; and I Fig. 2 is a diagram of the lens system of the tube of-Fig; l'in analogy to geometric optics.

Theparticl lar electron discharge device. chosen .forv

2 illustrating this invention is of the Iatron type comprising the tubular envelope 1 with the planar phosphor anode 2 across one end of the envelope and the flood gun structu fe 3 at the other end. The writing gun 4 is co centrically disposed at the flood gun-end of the tube. In the Iatron, the collector screen 5 and the storage target screen 6 are placed immediately in front of the phosphor anode. An electron charge image is written upon the storage target 6 by the writing gun 4. This charge image in turn modulates the low velocity electron flow from the flood gun 3, in proportion to the local target charge density at any particular point on the storage surface, thus controlling the position and current density of flood electrohsreaching the phosphor screen. The requirement that the flood electrons approach the collector screen and storage target normal to their plane gives rise to the invention.

This gun operates at zero (0) voltage on cathode acceleration of the beam to the electrodes primarily accomplished by the collector screen. According to this invention, three focusing electrodes 10, 11 and 12 are disposed concentric with the envelope between the gun and the planar electrodes. The potentials of these three electrodes are positive with respect to the cathode but the potential of the center electrode 11 is lower than the potentials of electrodes 10 and 12 so that in the plane 13 between the first two electrodes is produced a negative electrostatic lens. The relative potentials between electrodes 10 and 11 are so chosen that V the gun beam is sufficiently diverged. With this selection of potentials, accordingly, the electron lens at plane 14 is positive and conveges the electron beam. A second positiveelectron lens is between electrode 12. and the collector screen 5. This second positive lens is in the plane 16. By first initially diverging the beam at lens 13 to a diameter sufficient for obtaining a maximum display size, the positive lens at 14 still has low abberation, because the beam diameter at that lens is small as compared to the lens itself The positive lens at 16 has low aberration, because. its etfective object distance is at 0 thus reducing the angle to X The relative potentials of 10, 11 and 12 are easily adjusted so that the electrons are collimated and approach the planar electrodes 5 and 6 substantially normal to the plane of the electrodes.

More specifically, the tube embodying this invention shown in Fig. 1 comprises the tubular glass envelope 1. The glass plate 2 sealed in one end of the envelope is preferably of optical glass and is internally coated with a transparent film of electroluminescent film such 1 oxide, coated on the gun side of the screen.

' tion and yet collect secondary electrons from the storage target. Conventionally, the collector seal ring 15, collector screen 5, and the collector mounting ring carried by the envelope are operated at the highest positive potential within the lens system, for accelerating and focus, :1

Conventionally, the two planarelectrodes- 5 and 6 are supported on rings 5a and 6a, respectively,

fitted into the envelope and are mounted apart by thetj ing purposes.

ceramic insulating spacer 7. These screens are in turn mounted rigidly to the tube by metal tabs welded betweenthe ring 5a and the ring 15,'which is thermally sealed to the glass.

At the opposite endof the envelope, the writing'gun: 1' A is mounted withbeam acceleratingand focusingelec- Patented Dec. 15, 1959 Closely spaced in front of the phosphor anode is the storage target screen 6 with an electron transmissivity of, say, 30% to 50%, and with a layer of semi-conducting material such as silicon mon- V In front of the storage target 6 k is placed the collector screen 5 of sufficient transmissivity as to not impair the T680111.

correct for the keystone distortion to be expected from.

the eccentric writing gun. I

The flood gun 3 is of the modified Pierce type comprising the cathode sleeve 3a with the dish-shaped coated cathode suriace 35 disposed opposite the conical opening of the gun anodes 3c. The optics of the gun anode and cathode are such that a substantially rectilinear flow of electrons issue from the gun opening.

According to this invention, the electrons, after leaving the gun, are subjected to a negative field produced by concentric electrode The angle of divergence can be made sulficiently wide so that the electrons passing through plane 13 are further diverged and are substantially evenly distributed, and yet the divergence is not so great that the beam diameter equals the diameter of the lens thereby avoiding aberration. divergence is quite sensitive to the axial placement of The angle of the gun opening with respectto the neck of the envelope at the gun. That is, the proximity of the electrode 10 to the beam as it emerges fromthe gun is most sensitive to that portion of the electrode which is closest to the gun. a

Electrodes 11 and 12 are concentric with the envelope and are insulatingly spaced between electrode 10 and collector seal ring 15. In the embodiment.illustrated, the' electrodes 10, 11 and 12'are shownas carbon or aquadag coatings'on the inner surface offthe glass envelope.

Metal cylindrical electrodes insulatingly. spaced inthe envelope would be the full electrostatic equivalent.

Where the electrodes are; carbon coatings as shown,

lead in conductors 20, 21 and 22-are-conveniently sealed throughthe side of the envelope'and the inner ends thereof sprayed with the carbon coating material to make good electrical contactwith the lead-ius. in-23 is connected to the screen mounting 15.

Lead- According to an important feature of this. invention,

electrodes 10 and 12 are at about the same voltage, while the potential of electrode 11 is at a voltage below either 10 or 12. While moving from a positivefield to a. field" that is relatively negative, or less positive, across plane 13, the electrons enter a decelerating region and are diverged. While moving across plane 14, however, the electrons enter a relatively, more positive' field'and are converged. Referring to Fig. 2, ifthe actual cathode is at point 0 and the normal beam angle is x the lens 13*will increase the apparent beam angle represented'by.

lines 25. Electrons at the periphery of lens 14 will be at "point 0 with' the increased beam angle x This will increase spherical aberration if the beam diameter at 14 is large, nearing that of the lens. However, if the beam diameter is 'small, not too much aberration will result at lens 14. Further convergence by .lens 16 will fully collimate the beam withoutaberration due to its efiectively long object distance '0 The total aberration of a multiple-lens system is small if the sum of the aberrationsfrom each lens is -less than'that produced by a simple lens.

One voltage supply for -energizingthe focusing electrode is shown in Fig. 1. The otentiometers aretapped seen and will be refracted as though the cathode "were as shownwith ring 15 and the collector 5 connected to the highest positive potentialt half. thevolta'ge of the ring 15; while "electrode ltlfiis adjusted. by means of potentiometer "2.9 to operate at about one-quarter the ring 15 voltage.

inches, one and five-eighths (1%) inches and two (2) inches, the ring 15 and collector screen 5 should be operated in the to 250 volt range. When ring 15 was 200 volts, good results were obtained by operating electrode 12 at 100 volts, 10 at 30 volts and 11 at 20 volts. A wide variety of voltage combinations can be found for producing satisfactory results.

While the principles of the invention have been described in connection with specific apparatus, it is to be clearly understoodthatthis description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is: I v

1. In a storage tube, a planar electrode, an electron gun for providing a low velocity floodbeam of electrons, said gun being spaced from the planar electrode on a perpendicular line to the electrode, and a lens system between said gun and said electrode for uniformly flooding the electrode with collimated electrons from said gun including successively a first lens for diverging the electrons, a second .lens for converging the electrons, and a third lens for converging the electrons.

2. In a storage tube, a tubular envelope, a planar target electrode across one end of the envelope, an elec; tron gun at the other end of the envelope fordirecting; a lowvelocity flood beam of electrons'toward said target electrode; three end-to-end tubular electrodes concentric with the envelope between the gun and planar electrodes, separate lead-in conductors for. each of said tubular electrodes and saidYtarget electrode, a source ofope ratin g potential connected toeach of said lead-in conductors, the source connected to the lead-in conductor of the cen ter" tubular electrode being ata. potential below m stential of the sources connected to the lead-ins oflthe; other tubular electrodes so that the'electrostatic fields at opposite ends of the centerv electrode respectively diverge and converge the electron beam moving from theguni toward the target electrode, the source connected' to the lead in of'the tubular electrode adjacent said target elec trode being at a potential below the potentialof the. source connected to the lead-in of said target electrode so that the electrostatic .field between said target electrode and said adjacent tubular electrode further con verges the electron beam moving from the gun toward said target electrode. H

3. In combination in a flood gun-typetub, aplanar'target electrode, an electron gun spaced from and Iacingthe target electrode for directing a low -velocit y .flbod beam of electrons toward said target electrode, a negativeelece tron lens and adouble positive electron lensseqiieritially disposed between the gun and planar electrode. a

4; In combination in aiflood gun-type tubef'aftarget of extended surface area, an electron gun of the type spaced from and facing the target for prp ecting' a collimated beam of low 'velocity flood electrons toward said target, means for spreadingthe beam to uniformly flood said extended area comprising first jadiv'erging electrostatic lens in front of the gun, and neiit'a double' cona verging electrostatic lens downstream from the divrging 5 lens. I l

5:111 combination in a storage tube, a tubular glass envelope, a flattened target" electrode across one end of the envelope, an electron flood gun at the othef' 'end of said envelope, a collector electrode between" said flood gun and said target electrode and adjacent thelatte'fia coating of conductive inaterial on the innerwalhbf said envelope" substantially covering said envelope 'be't'we'en said gu'n and' said collector e'lec'trode, said coating being circumferentially divided into three mutually insulated tubular sections, separate lead-ins for each of' said sections and said collector electrode; avoltage source connected to each ofsaid lead-ins, the voltage source con} nected to the lead-in of'the intermediate one of said coati ing -sections bei'ng'hta w taltage belowthe voltages of the voltage sources connected to the lead-ins of the coating sections upstream and downstream from said intermediate coating section, the voltage source connected to the leadin of said collector electrode being at a voltage higher than the voltage source connected to the lead-in of the coating section adjacent said collector electrode, said adjacent coating section terminating short of said collector electrode.

UNITED STATES PATENTS Gray Aug. 18, 1942 Smith Dec. 27, 1955 Reed Apr. 10, 1956 Herman Apr. 30, 1957 Haefi Aug. 13, 1957 

